The present invention relates to solid-state exciter circuits for establishing arc discharges in igniter devices, and is directed more particularly to a solid-state exciter circuit for establishing arc discharges of improved repeatability and controllability and thereby increasing both the reliability and useful lives of igniter devices.
Energy generating systems which derive their energy from the combustion of fossil fuels all require igniters to ignite the fuel-air mixtures used therein as necessary to maintain the desired rate of energy output. Boilers, for example, use igniters to initiate the combustion of the heavy fuel oil and air mixtures used therein. Ground based turbines, on the other hand, use igniters to initiate the combustion of the natural gas and air mixtures commonly used therein. If the energy output of such systems is regulated by controlling combustion on a cycled or on-off basis, the igniter may be required to ignite the fuel-air mixture at the beginning of each combustion cycle. Additional ignitions may be required if the system is subject to "flame-outs" as a result of transient fluctuations in the rate of fuel and/or air flow.
In exciting the igniters used with such systems, it has long been known that reliable ignition requires the establishment of an arc discharge rather than a glow discharge between the electrodes of the igniter. This is because an arc discharge releases a large quantity of energy as a result of the high current that flows when even a small quantity of metal vapor is present in the ionized air between the igniter electrodes. A glow discharge, on the other hand, releases only a small quantity of energy because only a relatively low current flows when no metal vapor is present in the ionized air between the igniter electrodes.
It has also long been known that a high voltage must be applied between the electrodes of an igniter to initiate an arc discharge, but that relatively low voltages are sufficient to maintain such a current, once it has been established. As a result, it has become a common practice to excite igniters with a two stage ignition pulse that provides a first relatively high voltage at the low current levels that flow before an arc discharge begins, and a second relatively low voltage at the high current levels that flow after an arc discharge has begun.
One example of an ignition apparatus which provides an ignition pulse of the above-mentioned two stage type is described in U.S. Pat. No. 5,163,411 (Koiwa, et al.). In the latter ignition apparatus, a thyristor switch causes two capacitors to simultaneously discharge through respective parts of a primary winding to produce additive voltages and currents in a secondary winding. Because of the differing time constants of these two discharges, the ignition pulse has a high voltage early portion and a lower voltage later portion.
U.S. Pat. No. 5,215,066 (Narishige) describes a broadly similar circuit in which a thyristor switch causes two capacitors to discharge through a primary winding to cooperatively apply an ignition pulse to a spark plug. Because one of the capacitors does not begin to charge until after the other has begun to discharge, the time at which the latter is switched in can be delayed with respect to the former. This, together with the fact that the later discharging capacitor discharges through a current limiting coil, causes the ignition pulse to have the desired two-stage characteristic.
While ignition apparatuses of the above-discussed types are suitable for use with automobile engines, they are not well suited for use with high energy systems such as boilers and ground or air-based turbines. One reason is that automobile engines use a highly volatile fuel-air mixture which is easy to ignite and which supports a combustion that spreads so quickly through its combustion chamber that it is properly regarded as explosive. As a result, even ignition systems which produce relatively short or poorly shaped ignition pulses are adequate for use with automobiles.
In high energy combustion applications, on the other hand, the fuel-air mixtures are much more difficult to ignite and support a combustion that spreads much more slowly through its combustion chamber. As a result, the magnitude, shape and duration of the ignition pulses are much more important than in automotive applications. The sameness of the such pulses from pulse to pulse, i.e., their repeatability, is also much more important in high energy combustion applications than in automotive applications. Such pulses are consequently much more difficult to generate than automotive ignition pulses.
Another reason that ignition devices used in automotive applications are not well suited for high energy combustion applications is that automotive ignition systems do not make the avoidance of misfires an important consideration. This is because, if misfires occur because ignition pulses have the wrong magnitude or duration, the resulting damage is easily and inexpensively corrected. One need only replace one or more damaged spark plugs with inexpensive, easily installed new spark plugs. The adverse effects of such misfires are in any case limited to a single user or small group of users.
In high energy combustion applications, on the other hand, misfires are an important design consideration. One reason is that the igniters used in such systems are considerably more expensive than spark plugs. Another is that igniters handle much greater amounts of energy and consequently may be more seriously damaged by "weak" firings, misfirings and short circuits than spark plugs. The consequences of damage to an igniter may also be much more serious than damage to a spark plug, since the replacement of an igniter may require the shutdown of a system or engine that serves many people.
Because of the serious consequences than can result from the weak firing or misfiring of the igniters of high energy combustion systems, a number of attempts have been made to provide circuitry which can detect and compensate-for such firings. Two such attempts are described in U.S. Pat. Nos. 5,343,154 (Frus) and 5,399,942 (Frus). Such detecting-compensating circuits can, however, be complex and costly and can interfere with the desired normal operation of the exciter as a whole.
In view of the foregoing, it will be seen that a need has existed for an exciter circuit which generates ignition pulses which have a predictable and repeatable waveform, and which is able to detect weak firings or misfirings of igniters and to take prompt action to minimize the igniter damage caused thereby.